1. Field of the Invention
The present invention relates to a mass spectrometer and a method of mass spectrometry.
2. Discussion of the Prior Art
Organic molecules and biomolecules may be identified by a technique known as MS/MS using a tandem mass spectrometer. Parent ions of interest are selectively transmitted by an upstream mass filter and are then fragmented in a collision cell. The resulting fragment ions are then analysed by a mass analyser downstream of the collision cell.
Known tandem mass spectrometers commonly use a collision cell in which the selected precursor or parent ion are induced to fragment upon colliding with gas molecules in the collision cell. The most common form of collision cell is an enclosed chamber into which gas is introduced. The collision gas is commonly nitrogen or argon, although other gases such as air, helium, xenon, methane or a mixture of gases may be used. The gas pressure is typically in the range 10−3 mbar to 10−2 mbar.
The optimum collision energy for fragmentating ions depends upon a number of factors including the mass, charge, composition and internal energy of the ion to be fragmented and the mass of the collision gas. The optimum collision energy for collision induced fragmentation generally increases with the mass of the ion to be fragmented. For singly charged peptide ions formed using a MALDI source and subsequently cooled by collisions with the molecules of a background gas it has been empirically determined that the optimum collision energy (CE) voltage:
 CE≈0.05 m
where m is the mass of the parent ion in daltons. The kinetic energy of an ion is given by:   E  =                    mv        2            2        =          e      ⁢                           ⁢      V      where E is the ion energy, m is the mass, v is the velocity of the ion, e is electron charge and V is Volts. Accordingly:       v    2    =            2      ⁢                           ⁢      e      ⁢                           ⁢      V        m  In MKS units and where M is in daltons:       v    2    =                    2        ×        1.6        ×                  10                      -            19                                      1.67        ×                  10                      -            27                                ⁢                   ⁢          V      m        ⁢                   ⁢                  (                  m          ⁢                      /                    ⁢          s                )            2      since electron charge is 1.6×10−19 coulombs and 1 dalton is 1.67×10−27 kg. According to the empirically determined relationship for singly charged ions the optimum collision energy voltage is approximately equal to the mass in daltons divided by 20 and hence:       v    2    =                    2        ×        1.6        ×                  10                      -            19                                      1.67        ×                  10                      -            27                                ⁢          1      20        ⁢                   ⁢                  (                  m          ⁢                      /                    ⁢          s                )            2      thus: ν2≈107(m/s)2ν≈3000 m/s
Hence the optimum collision conditions are conventionally met when ions irrespective of their mass enter a collision cell having e.g. nitrogen or argon collision gas with a velocity of approximately 3000 m/s. Once the ions enter a conventional collision cell then they quickly lose their energy. The empirically determined optimum velocity of approximately 3000 m/s is not therefore an average velocity of the ions travelling through the collision cell but rather corresponds with the velocity that the ions should have upon initially entering the collision cell.
Conventionally it is known to accelerate ions having different masses so that the ions have substantially the same energy prior to entering a collision cell. However, it is not known to accelerate ions having different masses to have substantially the same velocity prior to entering a collision cell.
Conventional collision cell arrangements are therefore unable to fragment a relatively large number of ions having different masses all at substantially the same time and all at substantially the optimum collision energy. The collision energy must either be set at some compromise value which will tend to be less than optimum for some of the ions entering the collision cell or the ions must be arranged to have a collision energy which is progressively increased in a stepped or otherwise scanned manner over an appropriate range of energies. If the range of parent ion masses to be fragmented is relatively large, for example ranging from mass 500 to 2500 daltons, then it is apparent that the ions will be fragmented in a sub-optimal manner.
It is therefore desired to provide a mass spectrometer having an improved fragmentation device.